Sheet feeding device and image forming apparatus

ABSTRACT

A sheet feeding device including a belt including a dielectric material seamlessly wound about rollers and disposed opposite a top surface of a stack of multiple sheets placed on a sheet storage stand to attract a sheet from the stack of multiple sheets and convey the sheet, and a charger to form a predetermined charge pattern on a surface of the belt. A shaft of one of the rollers provided downstream from the other roller in a direction of sheet feed serves as a pivot of the belt about which the belt swings, and the belt swings around the pivot such that the surface of the belt facing the top surface of the stack of multiple sheets and a surface of the sheet storage stand facing the surface of the belt are substantially parallel to each other.

PRIORITY STATEMENT

The present patent application claims priority from Japanese Patent Application No. 2008-201628, filed on Aug. 5, 2008 in the Japan Patent Office, which is hereby incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

Illustrative embodiments described in this patent specification generally relate to a sheet feeding device to separate a stack of sheets and feed the sheets one by one, and an image forming apparatus including the sheet feeding device.

2. Description of the Related Art

Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction devices having two or more of copying, printing, scanning, and facsimile functions, typically form a toner image on a recording medium (e.g., a sheet) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of a latent image bearing member (e.g., a photoconductor); an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.

The image forming apparatuses further include a sheet feeding device to feed recording media such as a sheet. A friction paper feed method using a pickup member is widely employed in the sheet feeding device. Examples of the pickup member include a roller and a belt each including a material having a larger coefficient of friction such as rubber. However, although a configuration of the sheet feeding device can be simplified by employing the friction paper feed method, the pickup member needs to be pressed against the sheet using a spring or the like to obtain a larger fictional force. Because the coefficient of friction at the surface of the material having a larger coefficient of friction such as rubber is changed over time and by environmental factors, the friction paper feed method cannot stably provide a higher ability to reliably feed the sheet.

In recent years, a wider variety of recording media including special paper such as coated paper and label paper have been used as well as plain paper in the image forming apparatuses by diverse users, and it is expected that the number of different types of recording media demanded by users will further increase in the future. However, because the coefficient of friction at the surface of some types of special paper is extremely small, it is often difficult to separate the sheets of special paper from one another using friction alone. Further, peeling may occur at a release part of release paper when the sheets of release paper are frictionally separated from one another using a roller and a pressing member.

One possible solution to the above-described problems is to employ an air suction method in which negative pressure is generated by air suction to suction and convey the recording media. However, although providing a more reliable ability to feed the recording media compared to the friction paper feed method, the air suction method is noisy and causes an increase in size and costs of the sheet feeding device. Consequently, the air suction method is not practical for image forming apparatuses installed in an office or the like.

As another approach, a sheet feeding device including a seamless dielectric belt rotated in a direction of sheet feed has been proposed. The seamless dielectric belt contacts a top surface of a stack of sheets, and alternating charges (that is, electrical charges of alternating polarity) are applied to a surface of the seamless dielectric belt by a member. The member includes a charging unit to form an alternating charge pattern on the surface of the seamless dielectric belt and a neutralizing unit to de-charge, or neutralize, the surface of the seamless dielectric belt. The charges thus applied to the surface of the seamless dielectric belt generate an electric field that in turn generates an attractive electrostatic force that attracts the sheet to the surface of the seamless dielectric belt. Accordingly, a sheet placed at the top of the stack of sheets (hereinafter referred to as a top sheet) is separated from the rest of the stack of sheets and is conveyed in the direction of sheet feed.

However, for a certain period of time after the seamless dielectric belt contacts the top sheet the attractive force thus generated by the electric field tends also to act on multiple other sheets in the stack of sheets as well as the top sheet. Consequently, these multiple other sheets are also attracted to the seamless dielectric belt together with the top sheet, and all sheets are conveyed in the direction of sheet feed without being separated from one another, causing multiple sheet feeding.

To prevent multiple sheet feeding, a technique in which the sheets are separated from one another and conveyed one by one a predetermined period of time after the seamless dielectric belt contacts the top sheet has been proposed.

For example, a separation/conveyance device is used that includes an attraction/separation unit to alternately attract and support a top sheet placed at the top of the stack of sheets using an attractive force generated by a non-uniform electric field and separate the top sheet from the stack of sheets. The separation/conveyance device further includes a drive control unit to move the attraction/separation unit, a sheet conveyance unit to convey the top sheet thus separated, and an auxiliary separation unit to assist the attraction/separation unit to separate the top sheet from the stack of sheets. The drive control unit moves the attraction/separation unit from a leading edge side of the top sheet to a predetermined position to contact the top sheet, and stops the attraction/separation unit at that position. After a predetermined period of time elapses, the drive control unit moves the attraction/separation unit again. The auxiliary separation unit applies a predetermined force between the top sheet attracted to the attraction/separation unit and a second sheet, that is, a next sheet placed immediately beneath the top sheet, to separate the top sheet from the second sheet.

However, although it can prevent multiple sheet feeding, higher-speed sheet feeding cannot be achieved by the above-described separation/conveyance device due to the stop time involved.

Yet another approach is to measure a property (property value) of a target sheet to be separated from the stack of sheets to adjust the charge on the belt accordingly, that is, to control the amount of charges applied to the surface of the seamless dielectric belt, distance between each charge, and period of time to attract the target sheet to the surface of the seamless dielectric belt based on the property value thus measured.

However, because the target sheet first needs to be separated from the stack of sheets in order to measure the property value thereof, the above-described control cannot be performed on the target sheet itself separated from the stack of sheets. Further, the above-described control cannot be performed when sheets each having a different property value are included in the same stack of sheets.

Still yet another approach is to provide a prevention member to the seamless dielectric belt. The prevention member is either fixed on or provided movably to the seamless dielectric belt in a direction opposite the direction of sheet feed, with a sheet feed path therebetween.

For example, a sheet feeding device may include a pickup member contacting a top surface of the stack of sheets to convey the sheets in the direction of sheet feed. The pickup member includes a unit to apply alternating charges to the seamless dielectric belt. The unit has a charger to form a charge pattern on the surface of the seamless dielectric belt and a neutralizer to remove that charge. The pickup member further includes a prevention member to assist to separate the sheets from one another.

For another example, a sheet feeding separation device includes a sheet feeding member and a separation member provided opposite the sheet feeding member. Alternating charges are applied to both the sheet feeding member and the separation member to form an alternating charge pattern on a surface of each of the sheet feeding member and the separation member. Accordingly, the sheet is alternately attracted to and separated from the sheet feeding member and the separation member by an attractive force generated as Maxwell stress. The sheet feeding separation device further includes a seamless dielectric belt and a seamless belt serving as a prevention member provided opposite the seamless dielectric belt. Alternatively, a rubber friction pad may be provided as the prevention member.

However, because the attractive force is also applied to multiple other sheets placed beneath the top sheet as well as the top sheet, it is difficult to reliably separate the sheets from one another using the prevention member.

Published unexamined Japanese Patent Application No. (hereinafter referred to as JP-A-) 2002-211777 discloses an image forming apparatus including a sheet conveyance device including sheet supply means that attracts the sheets using electrostatic attraction to supply the sheets stored in a sheet storage to a conveyance path. The sheet conveyance device further includes separation means that uses electrostatic attraction to separate a target sheet to be conveyed from multiple other sheets supplied by the sheet supply means.

In order to reliably attract the sheet to the seamless dielectric belt, it is important to provide a wide contact area between the seamless dielectric belt and the sheet. At the same time, however, although reliable attraction of the sheet to the seamless dielectric belt is required to separate the sheets from one another, a leading edge of the sheet needs to be securely removed from the seamless dielectric belt once the sheets are separated from one another. Further, multiple sheets attracted to the seamless dielectric belt must be reliably separated from one another in order to achieve high-speed sheet feeding without causing multiple sheet feeding.

SUMMARY

In view of the foregoing, illustrative embodiments described herein provide a sheet feeding device to reliably separate a sheet from a stack of multiple sheets and feed the sheets one by one over time even when a wider variety of sheets is used, and an image forming apparatus including the sheet feeding device.

At least one embodiment provides a sheet feeding device including a belt including a dielectric material seamlessly wound about rollers and disposed opposite a top surface of a stack of multiple sheets placed on a sheet storage stand to attract a sheet from the stack of multiple sheets and convey the sheet, and a charger to form a predetermined charge pattern on a surface of the belt. A shaft of one of the rollers provided downstream from the other roller in a direction of sheet feed serves as a pivot of the belt about which the belt swings, and the belt swings around the pivot such that the surface of the belt facing the top surface of the stack of multiple sheets and a surface of the sheet storage stand facing the surface of the belt are substantially parallel to each other.

At least one embodiment provides an image forming apparatus including the sheet feeding device described above.

Additional features and advantages of the illustrative embodiments will be more fully apparent from the following detailed description, the accompanying drawings, and the associated claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the illustrative embodiments described herein and the many attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic view illustrating an example of a configuration of an image forming apparatus according to illustrative embodiments;

FIG. 2 is a perspective view illustrating an example of a sheet attraction/separation unit included in a sheet feeding device according to a first illustrative embodiment;

FIG. 3 is a schematic cross-sectional view illustrating the sheet attraction/separation unit illustrated in FIG. 2;

FIGS. 4A and 4B are schematic cross-sectional views illustrating operation of the sheet attraction/separation unit illustrated in FIGS. 2 and 3;

FIG. 5 is a schematic cross-sectional view illustrating a variation of the sheet attraction/separation unit included in the sheet feeding device according to the first illustrative embodiment;

FIGS. 6A to 6D are timing charts each illustrating a control sequence of an applied voltage and a frequency in the sheet feeding device;

FIG. 7 is a schematic cross-sectional view illustrating a sheet attraction/separation unit included in a sheet feeding device according to a second illustrative embodiment;

FIG. 8 is a schematic cross-sectional view illustrating a sheet attraction/separation unit included in a sheet feeding device according to a third illustrative embodiment;

FIG. 9 is a schematic cross-sectional view illustrating a sheet attraction/separation unit included in a sheet feeding device according to a fourth illustrative embodiment;

FIG. 10 is a schematic cross-sectional view illustrating a sheet attraction/separation unit included in a sheet feeding device according to a fifth illustrative embodiment;

FIG. 11 is a schematic cross-sectional view illustrating a sheet attraction/separation unit included in a sheet feeding device according to a sixth illustrative embodiment;

FIG. 12 is a perspective view illustrating a separation unit included in the sheet feeding device according to the sixth illustrative embodiment; and

FIG. 13 is a schematic cross-sectional view illustrating a sheet attraction/separation unit included in a sheet feeding device according to a seventh illustrative embodiment.

The accompanying drawings are intended to depict illustrative embodiments and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In describing illustrative embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result.

Reference is now made to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views.

FIG. 1 is a schematic view illustrating an example of a configuration of a copier serving as an image forming apparatus 1 according to illustrative embodiments. The image forming apparatus 1 includes a document reading unit 2, an image forming unit 3, and a sheet feeder 4. In the image forming apparatus 1, the image forming unit 3 and the sheet feeder 4 are separately formed, and are detachably attachable to each other.

The sheet feeder 4 includes a sheet feeding device 5. The sheet feeding device 5 includes a sheet attraction/separation unit 7, a charging roller 8, and a friction pad 9. The sheet attraction/separation unit 7 contacts a top surface of a stack of sheets 6 stored in a sheet feed cassette, not shown, to attract a top sheet 6 a placed at the top of the stack of sheets 6, and separates the top sheet 6 a from the stack of sheets 6. According to illustrative embodiments, the sheet attraction/separation unit 7 is detachably attachable to the sheet feeder 4.

Specifically, the sheet attraction/separation unit 7 attracts the stack of sheets 6 including the top sheet 6 a placed at the top thereof, and the friction pad 9 separates the top sheet 6 a from the stack of sheets 6. Accordingly, the sheets included in the stack of sheets 6 are fed to a pair of registration rollers 11 one by one. Each sheet is then conveyed to a transfer device 12 from the pair of registration rollers 11, and a toner image formed by the image forming unit 3 is transferred onto the sheet in the transfer device 12. Thereafter, the sheet having the toner image thereon is conveyed to a fixing device 13, and heat and pressure are applied to the sheet to fix the toner image to the sheet. The sheet having the fixed toner image thereon is discharged by a discharge roller 14 to a discharge tray 15.

It is to be noted that the sheet feeding device 5 according to illustrative embodiments is also applicable to image forming apparatuses other than the image forming apparatus 1 employing electrophotography, such as inkjet type image forming apparatuses. Further, the sheet feeding device 5 according to illustrative embodiments is applicable to facsimile machines, printers, multifunction devices having two or more of copying, printing, scanning and facsimile functions, and so forth, as well as copiers as described above. It is also to be noted that the sheet feeding device 5 is also known as a sheet feeding/separation device.

A description is now given of the sheet attraction/separation unit 7 included in the sheet feeding device 5 according to a first illustrative embodiment with reference to FIG. 2.

The sheet attraction/separation unit 7 includes a drive roller 22, a driven roller 23, and a seamless belt 19 including a dielectric material stretched between the drive roller 22 and the driven roller 23. Specifically, the belt 19 includes a dielectric material having a resistivity of 10⁸ Ωcm or greater. An example of the dielectric material includes a polyethylene terephthalate film, having a thickness of about 100 μm. The sheet attraction/separation unit 7 further includes a bottom plate 28 and an insulating sheet 29 provided on the bottom plate 28. The insulating sheet 29 enables the bottom plate 28 to be formed of a metal material, providing higher rigidity to the bottom plate 28. As a result, even a last sheet placed at the bottom of the stack of sheets 6 can be reliably fed to the image forming unit 3.

FIG. 3 is a schematic cross-sectional view illustrating the sheet attraction/separation unit 7 illustrated in FIG. 2. As illustrated in FIG. 3, the belt 19 is double-layered, including a top layer 19 a and a bottom layer 19 b. The top layer 19 a includes a dielectric material having a resistivity of 10⁸ Ωcm or greater, and the bottom layer 19 b includes a conductive material having a resistivity of 10⁶ Ωcm or less. Because a charging electrode 21 can be used as a counter electrode grounded to the bottom layer 19 b, the charging electrode 21 may be provided at any position contacting the surface of the belt 19, that is, the top layer 19 a. According to the first illustrative embodiment, the charging electrode 21, including a roller connected to an AC power supply 24, is provided such that the charging electrode 21 contacts the belt 19 at a position where the belt 19 is wound around the drive roller 22. The stack of sheets 6 is positioned so as to obtain an area of the sheet attracted to the belt 19 sufficient for reliable removal. The drive roller 22 includes a conductive rubber layer having a resistivity of about 10⁶ Ωcm on a surface thereof. The driven roller 23 is a metal roller. Both the drive roller 22 and the driven roller 23 are grounded. An upper guide plate 26 and a lower guide plate 27 each restricting conveyance of the sheets are provided on a downstream side from the belt 19 relative to a direction of sheet feed. Specifically, the surface of the belt 19 facing the stack of sheets 6 is provided in parallel to a surface of the bottom plate 28 on which the stack of sheets 6 is placed. Further, the sheet attraction/separation unit 7 is provided on a downstream side from a leading edge of the top sheet 6 a to be separated from the stack of sheets 6 relative to the direction of sheet feed, such that the leading edge of the top sheet 6 a never contacts the belt 19 including a dielectric material. As a result, the top sheet 6 a can be effectively removed from the belt 19 using a curvature of the belt 19.

The belt 19 swings around a shaft 22 a of the drive roller 22 as a pivot as illustrated in FIG. 4A, and is positioned at a front part of an upper surface of the top sheet 6A of the stack of sheets 6 to contact the top sheet 6A. The pivot is positioned such that the top surface of the stack of sheets 6 on the bottom plate 28 pushed upward by a spring 37 is on a tangential line to the curvature of the belt 19 on the drive roller 22 side.

In addition, one side of the belt 19, that is, the driven roller 23 side in the case of FIGS. 4A and 4B because the shaft 22 a of the drive roller 22 serves as the pivot as described above, swings to maximize an area of contact between the stack of sheets 6 and the belt 19. Because the sheet attraction/separation unit 7 is designed to have an attractive force sufficient to remove the top sheet 6A, the spring 37 only pushes the stack of sheets 6 upward via the bottom plate 28 such that the stack of sheets 6 touches the sheet attraction/separation unit 7. Further, a force that attracts a next sheet to be fed can be increased, and a space is formed so that the sheets are reliably separated from one another.

The charging electrode 21 is provided on the drive roller 22 side so that a load applied to the stack of sheet 6 from the sheet attraction/separation unit 7 swung around the shaft 22 a of the drive roller 22 by its own weight is reduced, causing a decrease in a frictional force between the top sheet 6 a and a second sheet 6 b, that is, the second sheet from the top of the stack of sheets 6.

Further, returning to FIG. 3, the friction pad 9 in the first illustrative embodiment engages the sheet attraction/separation unit 7, so that the friction pad 9 swings together with the sheet attraction/separation unit 7. As a result, an angle of approach of the sheet to the friction pad 9 can be kept constant. In place of the friction pad 9, for example, a retard roller may be used. As illustrated in FIG. 3, the friction pad 9 and the lower guide member 27 are pushed upward by a spring 38.

When driving of the drive roller 22 is stopped, a free edge side of the bottom plate 28 is pushed upward by the spring 37 as illustrated in FIG. 4B, and one edge side of the sheet attraction/separation unit 7 is moved upward due to swinging movement. It is to be noted that a stopper, not shown, that keeps the sheet attraction/separation unit 7 swingingly moved upward horizontal may be preferably provided. Specifically, when a free edge of the bottom plate 28 pushed upward contacts the pivot of the belt 19, only the weight of the belt 19 keeps the belt 19 in contact with the stack of sheets 6. Accordingly, a pressure applied to the stack of sheets 6 can be kept constant regardless of the number of sheets in the stack of sheets 6, and a frictional force between each sheet in the stack of sheets 6 can be reduced. Further, the area of contact between the belt 19 and the stack of sheets 6 can be effectively used, providing the ability to consistently separate the stack of sheets 6 from one another without causing multiple sheet feeding and moreover without wrinkling the next sheet.

FIG. 5 is a schematic cross-sectional view illustrating a variation of the sheet attraction/separation unit 7. As illustrated in FIG. 5, the stack of sheets 6 on the bottom plate 28 can be moved upward or downward by a pushing device 40, and a sensor 30 is provided to detect the upper surface of the top sheet 6 a at the top of the stack of sheets 6. Accordingly, a distance and a pressure between the belt 19 and the top sheet 6 a can be appropriately controlled.

It is to be noted that the AC power supply 24 may supply a direct current varied between alternating high and low electric potentials as well as an alternating current. In the first illustrative embodiment, an alternating current having an amplitude of 4 KV is applied to the surface of the belt 19. FIGS. 6A to 6D are timing charts each showing a control sequence of an applied voltage and a frequency in the sheet feeding device 5.

The belt 19 having a charge pattern thereon contacts the front part on the upper surface of the top sheet 6 a at a position where the belt 19 is wound around the driven roller 23. Accordingly, the Maxwell stress acts on the top sheet 6 a due to a non-uniform electric field formed by the charge pattern on the surface of the belt 19. As a result, only the top sheet 6 a is attracted to the belt 19 and is conveyed in the direction of sheet feed to the pair of registration rollers 11 through the upper guide member 26 and the lower guide member 27. Thereafter, the top sheet 6 a is further conveyed to the image forming unit 3 by the pair of registration rollers 11. The force that attracts the top sheet 6 a generated by the charge pattern as described above also acts on the second sheet 6 b and some other sheets beneath the second sheet 6 b for a certain period of time from when the top sheet 6 a is attracted to the belt 19. However, after the certain period of time has elapsed, the force acts only on the top sheet 6 a so that the second sheet 6 b and the other sheets beneath the second sheet 6 b are no longer attracted to the belt 19. Accordingly, the top sheet 6 a can be separated from the stack of sheets 6 without using a prevention member with enough time.

As illustrated in FIG. 2, the insulating sheet 29 is provided on the contact surface between the bottom plate 28 and the belt 19 to generate a force that attracts the last sheet, that is, the sheet at the bottom of the stack of sheets 6. Further, as illustrated in FIG. 3, a neutralizing electrode 25 connected to a neutralizing power supply 16 serving as an alternate power supply is provided on an upstream side from the charging electrode 21 relative to the direction of sheet feed and a downstream side from a position where the sheet is separated from the belt 19 relative to the direction of sheet feed. The neutralizing electrode 25 contacts the belt 19 or is provided near the belt 19. In the sheet attraction/separation unit 7 according to the first illustrative embodiment, the AC power supply 24 and the neutralizing power supply 16 are controlled such that the force on the belt 19 that attracts the sheet is eliminated at a position where the leading edge of the, sheet contacts the pair of registration rollers 11. Therefore, the sheet sandwiched by the pair of registration rollers 11 is conveyed only by the pair of the registration rollers 11 without being affected by the belt 19.

A description is now given of operations of the sheet feeding device 5. When an electromagnetic clutch is turned on in response to a sheet feed signal, the drive roller 22 is driven to rotate the belt 19. Alternating charges are applied to the belt 19 from the AC power supply 24 through the charging electrode 21 so that a charge pattern alternating with a pitch corresponding to an AC power frequency and a rotation speed of the belt 19 is formed on the surface of the belt 19. It is to be noted that the pitch is preferably set in a range from 5 mm to 15 mm.

A linear velocity of the pair of registration rollers 11 is the same as that of the belt 19. When the pair of registration rollers 11 is driven intermittently with intervals, the belt 19 is driven intermittently. The belt 19 is separated from the stack of sheets 6 before a rear edge of the top sheet 6 a reaches a position facing the driven roller 23 so that the second sheet 6 b is not attracted to the belt 19. The sheet is separated from the belt 19 at the drive roller 22 by the curvature of the belt 19 against the drive roller 22 thereat, and further separated from the belt 19 using the upper guide plate 26 and the lower guide plate 27 illustrated in FIG. 3.

According to the first illustrative embodiment, the top sheet 6 a is attracted to the belt 19 having the alternating charge pattern thereon and is separated from the stack of sheets 6. Thereafter, the top sheet 6 a passes through the friction pad 9 using friction. Accordingly, even when multiple sheets are attracted to the belt 19 at the same time, those sheets are separated one by one by the friction pad 9. As a result, the sheets are reliably separated from one another and fed one by one, preventing multiple sheet feeding.

It is preferable to provide a cleaning mechanism for the belt 19 in order to prevent paper dust and other foreign substances attached to the belt 19 from disturbing attraction of the sheets to the belt 19.

The sheet feeding device 5 according to the first illustrative embodiment can handle a wider variety of sheets regardless of a coefficient of friction thereof, and provide an image forming apparatus including a sheet feeding device having an ability to reliably separate the sheets one by one.

A description is now given of the sheet feeding device 5 according to a second illustrative embodiment. FIG. 7 is a schematic cross-sectional view illustrating the sheet attraction/separation unit 7 included in the sheet feeding device 5 according to the second illustrative embodiment. As illustrated in FIG. 7, a separation pick 33 is provided so that the sheet is further reliably separated from the belt 19 using the separation pick 33 as well as the curvature of the belt 19 at the drive roller 22. The rest of the configuration and operations of the sheet feeding device 5 according to the second illustrative embodiment are same as those of the sheet feeding device 5 according to the first illustrative embodiment, and a description thereof is omitted.

FIG. 8 is a schematic cross-sectional view illustrating the sheet attraction/separation unit 7 included in the sheet feeding device 5 according to a third illustrative embodiment. In the foregoing illustrative embodiments, the sheet attraction/separation unit 7 is provided on a downstream side from the leading edge of the top sheet 6 a to be separated from the stack of sheets 6 relative to the direction of sheet feed. By contrast, according to the third illustrative embodiment, the sheet attraction/separation unit 7 is provided on an upstream side from the leading edge of the top sheet 6 a relative to the direction of sheet feed. As a result, a number of drive transmission parts can be reduced compared to the case in which the drive roller 22 is provided on the free edge side of the bottom plate 28, and the weight of the belt 19 can be reduced.

FIG. 9 is a schematic cross-sectional view illustrating the sheet attraction/separation unit 7 included in the sheet feeding device 5 according to a fourth illustrative embodiment. As illustrated in FIG. 9, in the sheet feeding device 5 according to the fourth illustrative embodiment, a separation unit 31 is provided on a downstream side from the sheet attraction/separation unit 7 relative to the direction of sheet feed. The separation unit 31 includes a paper feed roller 18 and a roller member 34 serving as a prevention member provided facing the paper feed roller 18. A one-way clutch, not shown, is provided to the roller member 34 to prevent the roller member 34 from rotating together with conveyance of the sheets. The roller member 34 is pressed against the paper feed roller 18 by a spring 34a. Specifically, the roller member 34 is pressed against the paper feed roller 18 at a very small pressure of, for example, 500 gf or less. When multiple sheets including the top sheet 6 a and the second sheet 6 b are attracted and conveyed by the belt 19, the second sheet 6 b is separated from the top sheet 6 a by friction between the second sheet 6 b and the roller member 34, and conveyance of the second sheet 6 b is stopped at the roller member 34. Because the pressure applied to the paper feed roller 18 from the roller member 34 is very small, conveyance of the top sheet 6 a frictionally supported by the paper feed roller 18 is not prevented. More specifically, a coefficient of friction between the roller member 34 and the top sheet 6 a is greater than a coefficient of friction between the top sheet 6 a and the second sheet 6 b. Accordingly, only the top sheet 6 a is supported by the paper feed roller 18, and is conveyed to a pair of conveyance rollers 17 provided on a downstream side from the paper feed roller 18 relative to the direction of sheet feed.

FIG. 10 is a schematic cross-sectional view illustrating the sheet attraction/separation unit 7 included in the sheet feeding device 5 according to a fifth illustrative embodiment. As illustrated in FIG. 10, a roller member 35 including a torque limiter is provided facing the paper feed roller 18 in the separation unit 31. The torque limiter causes the roller member 35 to rotate in a direction indicated by an arrow a in FIG. 10 when multiple sheets including the top sheet 6 a and the second sheet 6 b are conveyed between the paper feed roller 18 and the roller member 35 to stop the conveyance of the second sheet 6 b at the roller member 35 and separate the second sheet 6 b from the top sheet 6 a. By contrast, the roller member 35 is rotated in a direction indicated by an arrow b in FIG. 10 along with rotation of the paper feed-roller 18 by a friction when only the top sheet 6 a is conveyed between the paper feed roller 18 and the roller member 35. The roller member 35 is pressed against the paper feed roller 18 by a spring 35 a. The rest of the configuration and operations of the sheet feeding device 5 according to the fifth illustrative embodiment are same as those of the sheet feeding device 5 according to the fourth illustrative embodiment, and a description thereof is omitted.

In the sheet feeding device 5 according to the fifth illustrative embodiment, the sheet attracted to the belt 19 is reliably separated from the belt 19 by the roller member 35 at a predetermined position so that the leading edge of the sheet is reliably guided to the separation unit 31. Because the leading edge of the top sheet 6 a at the top of the stack of sheets 6 does not contact the belt 19 as described in the first illustrative embodiment, the leading edge of the top sheet 6 a is reliably guided to the separation unit 31 without using secondary means such as a separation pick.

FIG. 11 is a schematic cross-sectional view illustrating the sheet attraction/separation unit 7 included in the sheet feeding device 5 according to a sixth illustrative embodiment. FIG. 12 is an enlarged perspective view illustrating the separation unit 31 included in the sheet feeding device 5 according to the sixth illustrative embodiment. As illustrated in FIGS. 11 and 12, the separation unit 31 includes the friction pad 9 including a friction member 36 and a bracket 44. The friction member 36 attached to the bracket 44 is provided facing the paper feed roller 18, and is pressed against the paper feed roller 18 at a very small pressure by a spring 38. The rest of the configuration and operations of the sheet feeding device 5 according to the sixth illustrative embodiment are same as those of the sheet feeding device 5 according to the fifth illustrative embodiment, and a description thereof is omitted. According to the sixth illustrative embodiment, an angle of approach of the sheet to the friction pad 9 from the belt 19 can be kept constant, reliably providing the ability to separate the sheets from one another.

FIG. 13 is a schematic cross-sectional view illustrating the sheet attraction/separation unit 7 included in the sheet feeding device 5 according to a seventh illustrative embodiment. As described above, in the foregoing illustrative embodiments, the sheet is separated from the belt 19 by the curvature of the belt 19 at the drive roller 22. By contrast, in the seventh illustrative embodiment, the sheet is separated from the belt 19 using only the upper guide plate 26. Because the sheet is separated from the surface of the belt 19 without using a separation pick, the number of components can be reduced.

Further, in the seventh illustrative embodiment, a friction pad system is employed in the separation unit 31. Accordingly, a configuration of the separation unit 31 can be simplified, reducing costs and a number of components.

An image forming apparatus including the sheet feeding device 5 according to the foregoing illustrative embodiments can handle a wider variety of sheets regardless of a coefficient of friction thereof, and reliably separate the sheets from one another.

It is to be noted that illustrative embodiments of the present invention are not limited to those described above, and various modifications and improvements are possible without departing from the scope of the present invention. It is therefore to be understood that, within the scope of the associated claims, illustrative embodiments may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of the illustrative embodiments. 

1. A sheet feeding device, comprising: a belt comprising a dielectric material seamlessly wound about rollers and disposed opposite a top surface of a stack of multiple sheets placed on a sheet storage stand to attract a sheet from the stack of multiple sheets and convey the sheet; and a charger to form a predetermined charge pattern on a surface of the belt, wherein: a shaft of one of the rollers provided downstream from the other roller in a direction of sheet feed serves as a pivot of the belt about which the belt swings; and the belt swings around the pivot such that the surface of the belt facing the top surface of the stack of multiple sheets and a surface of the sheet storage stand facing the surface of the belt are substantially parallel to each other.
 2. The sheet feeding device according to claim 1, wherein a leading edge of the stack of multiple sheets is positioned opposite a bottom part of a curved portion of the belt on a downstream side relative to the direction of sheet feed.
 3. The sheet feeding device according to claim 1, wherein the charger is provided adjacent to the pivot of the belt.
 4. The sheet feeding device according to claim 1, wherein a pivot-side roller around which the belt is wound is rotatably driven.
 5. The sheet feeding device according to claim 1, further comprising an upper guide plate provided downstream from a leading edge side of the stack of multiple sheets placed on a sheet storage stand in the direction of sheet feed, wherein a surface of a curved portion of the belt provided downstream in the direction of sheet feed intersects with the upper guide plate.
 6. The sheet feeding device according to claim 1, further comprising a biasing member to lift a free edge of the belt opposite an end of the belt at the pivot.
 7. The sheet feeding device according to claim 1, further comprising a unit to keep the belt horizontal at an uppermost position at a top of an arc through which the belt swings.
 8. The sheet feeding device according to claim 1, further comprising an insulating sheet disposed on a bottom plate portion of the sheet feeding device, wherein at least a portion of the insulating sheet disposed near the surface of the sheet storage stand contacting the belt comprises an insulating material.
 9. The sheet feeding device according to claim 1, further comprising a unit to feed each sheet from the stack of multiple sheets using frictional force between the unit and sheets at a position where the sheets are separated from the belt.
 10. The sheet feeding device according to claim 1, further comprising a separation unit contacting the belt, wherein the separation unit swings together with the belt.
 11. The sheet feeding device according to claim 10, wherein the separation unit comprises a separation pick.
 12. The sheet feeding device according to claim 10, wherein the separation unit comprises a friction pad.
 13. The sheet feeding device according to claim 10, wherein the separation unit comprises a separation roller.
 14. The sheet feeding device according to claim 13, wherein the separation roller is reversely rotatable.
 15. An image forming apparatus comprising a sheet feeding device, the sheet feeding device comprising: a belt comprising a dielectric material seamlessly wound about rollers and disposed opposite a top surface of a stack of multiple sheets placed on a sheet storage stand to attract a sheet from the stack of multiple sheets and convey the sheet; and a charger to form a predetermined-charge pattern on a surface of the belt, wherein: a shaft of one of the rollers provided downstream from the other roller in a direction of sheet feed serves as a pivot of the belt about which the belt swings; and the belt swings around the pivot such that the surface of the belt facing the top surface of the stack of multiple sheets and a surface of the sheet storage stand facing the surface of the belt are substantially parallel to each other. 